Problem: The lifespans of snakes in a particular zoo are normally distributed. The average snake lives $21.8$ years; the standard deviation is $3.8$ years. Use the empirical rule (68-95-99.7%) to estimate the probability of a snake living longer than $14.2$ years.
Solution: $21.8$ $18$ $25.6$ $14.2$ $29.4$ $10.4$ $33.2$ $95\%$ $2.5\%$ $2.5\%$ We know the lifespans are normally distributed with an average lifespan of $21.8$ years. We know the standard deviation is $3.8$ years, so one standard deviation below the mean is $18$ years and one standard deviation above the mean is $25.6$ years. Two standard deviations below the mean is $14.2$ years and two standard deviations above the mean is $29.4$ years. Three standard deviations below the mean is $10.4$ years and three standard deviations above the mean is $33.2$ years. We are interested in the probability of a snake living longer than $14.2$ years. The empirical rule (or the 68-95-99.7 rule) tells us that $95\%$ of the snakes will have lifespans within 2 standard deviations of the average lifespan. The remaining $5\%$ of the snakes will have lifespans that fall outside the shaded area. Because the normal distribution is symmetrical, half $({2.5\%})$ will live less than $14.2$ years and the other half $({2.5\%})$ will live longer than $29.4$ years. The probability of a particular snake living longer than $14.2$ years is ${95\%} + {2.5\%}$, or $97.5\%$.